Leaky coaxial cable

ABSTRACT

A leaky coaxial cable wherein the inner conductor is helically wound with its winding direction reversed alternately at a predetermined interval which is approximately equal to 1/2 of the wave length of the operating frequency and the outer conductor is a conductor tube having a gap or straight array of slots provided along the cable axis. The leaky coaxial cable of the present invention, which is installed horizontally, leaks out a vertically polarized leaky wave which is very suitable for use in radio communication systems.

United States Patent Miyamoto et al.

1 Sept. 30, 1975 1 LEAKY COAXlAL CABLE [75] Inventors: Yoshio Miyamoto; Toshiyuki Ikemiya, both of Osaka. Japan [73] Assignee: Sumitomo Electric Industries. Ltd..

Osaka Japan [22] Filed: Mar. 6, 1974 [21 Appl. No: 448,680

30 Foreign Application Priority Data Mar. 13, 1973 Japan 48-29254 [52] U.S. Cl 333/97 R; 333/96; 343/770 [51] Int. Cl. H01? 3/06: H0 10 13/20; 'HOlQ 13/22 [58] Field of Searchmm... 333/84 R. 96 97 R. 95 S: 343/767-768. 770771; 325/51 (56] I References Cited UNITED STATES PATENTS 3,781.725 12/1973 Yoshida et al 333/84 R Primary E.\'umii1er.lames W. Lawrence Ass/slant E.\'unzincrMarvin Nussbaum Attorney. Agent. or FirmCarothers and Carothers I57] ABSTRACT A leaky coaxial cable wherein the inner conductor is helically wound with its Winding direction reversed alternately at a predetermined interval which is approximately equal to H2 of the wave length of the operating frequency and the outer conductor is a conductor tube having a gap or straight array of slots provided along the cable axis. The leaky coaxial cable of the present invention. which is installed horizontally, leaks out a vertically polarized leaky wave which is very suitable for use in radio communication systems.

4 Claims, 5 Drawing Figures US. Patent Sept. 30,1975

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. LEAKY COAXIAL CABLE BACKGROUND or THE INVENTION 1. Field of'the. Invention The present invention relates to a leaky coaxial cable used forradidcomm-unication systems.

In a radio communication systemwhich includes in its serviceiarea an area wherein the radiation field is extremely weak or not presenttfor example. in the interior of a tunnel, under a street or in the interior of a building), a leaky coaxial cable is'ordinally used to overcome the difficulty. i

One such leaky coaxial cable conventionally used is shown inFlG. 1. In-FIG. 1, 1 denotes an outer corrugated conductor generally having slots 2,, 2 in the axial direction with apredetermined slot periodicity, each slot being slanted with a predetermined angle with respect to the cable axis, and 5. denotes an inner cond t 1 I '5 Y The radio communication wave energy propagates in the coaxial cable and apart of it leaks out from'the slot array of the horizontally installed'leaky coaxial cable.

The leaked wave is polarized vertically andcouples with the antenna of a receiver. Y

2. Description of the Prior Art' The essential point in the construction of a cable of this kind is that the slot shape and'slot pitch must be made 'with a high degree of precision. The outer conductor is usually made of thin-corrugated aluminum plate. In thelnianufa'c'turin'g method for such aleaky 'coaxial cablei 'th'e r e" are known processes of perforating the slots at a predetermined interval in a thin metal band. of corrugating the plate in such a manner that straight small parallel ridges are produced in the direction lateral to the metal band. In such a process, the adjacent slots must have a predetermined precise interval and each of the slots must have a predetermined slot angle with respect to the cable axis with the band in its corrugated state. The corrugated plate is bent longitudinally so as to make a tube including the inner conductor, and a plastic sheath is applied over the coaxial structure.

Since the slot array of the final product needs a very high precision in its slot shape, interval between adjacent slots and the slant angle of the slot with respect to the cable axis of the final product, excessive difficulty is incurred in its manufacturing processes. Accordingly, the conventional leaky coaxial cable as shown in FIG. 1 has a great disadvantage in manufacturing cost.

SUMMARY OF THE INVENTION It is the object of this invention to provide a leaky coaxial cable which has a straight gap or straight array of slots in the outer conductor for the transmission of a leaky wave along the cable axis and an inner conductor helically wound with its winding direction reversed alternately at a preset interval. It thus overcomes the aforementioned disadvantage. The leaky coaxial cable of the present invention is manufactured very economically. The transmission properties of the cable of the present invention also are not different from those of the conventional ones. The leaky coaxial cable may be broadly used for service in weak field areas of radio communication systems, for example, CATV systems and vehicle communication systems.

The excellent features and advantages of the present invention will be apparent from the following detailed description and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 isjan obliqueview with portions removed of a leaky cable of the prior art.

FIG. 2 is an oblique view with portions removed of the leaky coaxial cable of the present invention.

FIG. 3 (a) is an oblique view ofthe inner conductor of the leaky coaxial cable with the current distribution thereon diagrammatically illustrated.

- FIG. 3 (b) is an oblique view of the outer conductor of the leaky coaxial cable of the present invention with the current distribution thereon diagrammatically illustrated. FIG. 3 (c) is an oblique view illustrating another type of outer conductor for the leaky coaxial cable of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 2, a slant view of the leaky coaxial cable is shown with the outer conductor 3 having gap means provided by gap 4 is continuous in the axial di-' rection of the cable and with an inner conductor 5 which is made by helically winding a suitable strip conductor and reversing its winding direction alternately at a predetermined periodicity d.

FIGS. 3 (a) and (b) show drawings for assisting in the explanation the principle of the leaky coaxial cable of the present invention. FIG. 3 (a) shows an inner conductor 5' which is divided into sections 5,, 5 each of a predetermined length d. In each section, the conductor wire is wound helically at the same pitch and its winding direction is reversed at each adjacent section. The length d of each section is selected to be approximately one-half of the wave length of the operating frequency in the coaxial cable. The currents distributed on each section 5,, 5 are shown by 6,, 6 in FIG. 3 ((1). Since the length of each section is approximately equal to one-half of the wave length in the coaxial cable, the current directions 6,, 6 are reversed from section to section. Accordingly, the magnetic fields generated by the currents 6,, 6 at sections 5,, 5 are shown by 7,, 7 respectively. The directions of the magnetic fields 7,, 7 are not reversed from section to section, because the direction of the current is reversed from section to section but the direction of winding of the helical inner conductor is also reversed from section to section.

FIG. 3 (b) shows an outer conductor 3 having gap means provided by gap 4 extending straight in the axial direction in parallel with the cable axis. The width of the gap 4 is sufficiently smaller than the operating wave length.

The magnetic fields 7,, 7 induce respectively induced currents 8,, 8 in the outer conductor and at the gap 4 flowing circumferentially in the same direction. The induced currents 8,, 8 at the gap 4 radiates a circumferentially polarized wave which is very suitable for a radio communication system. There are, however, no induced currents in the vicinity of the points where the winding direction of the inner conductor is reversed.

In view of this fact, the leaky coaxial cable which consists of the combination of the inner conductor as shown in FIG. 3 (a) and the outer conductor with gap means provided by a straight array of slots extending in parallel with the cable axis as shown in FIG. 3 (c), is equally as good for use in place of such leaky coaxial cables having the outer conductor as shown in FIG. 3(b).

In FIG. 3(a), 9 9 indicate the straight slots posi tioned in the axial direction and corresponding to the sections 5,, 5 of the inner conductor respectively. The length of each slot is slightly shorter than the periodicity d and the width of each of the slots is the same and sufficiently smaller than the operating wavelength.

The leaky coaxial cable according to the present invention can be used effectively for mobile radio bands utilizing VHF, UHF, etc. Especially in the case of long wavelength bands, however, a flexible body of magnetic material or of insulating material therein having a magnetic material may be used as a winding mandrel for the central helical conductor with the object of further improving the excitation efficiency of the opening of the outer conductor by enhancing the contraction effect of the wavelength in the waveguide.

As is clear from the above description, the present invention makes it possible to obtain a leaky coaxial cable which leaks out a vertically polarized wave which is very suitable for use in a radio communication system. The structure comprises an inner conductor of a helically wound wire whosc winding direction is reversed alternately at a predetermined length and an outer conductor having an opening such as either a straight gap or straight array of slots in the direction of the cable axis.

The leaky coaxial cable of the present invention has a very stable and simple mechanical structure so that it has good productivity with high precision in manufacture and is easy to handle for installation and maintenance.

For use in a radio communication system, the leaky coaxial cable further makes it very easy to regulate the amount of leaky wave energy per unit length radiating from the opening by partially shielding the radiation area of the opening with a conductor plate such as a metal plate. Needless to say, the helical structure of the inner conductor may be made up of two or more parallel conductor wires or strips instead of a single wire or strip as shown in FIG. 3 (a).

What we claim is:

1. A leaky coaxial cable comprising an inner conductor of at least one helically wound conductor with its winding direction reversed alternately at a predetermined interval approximately equal to one-half of the wave length of the operating frequency, and a coaxial outer conductor having gap means extending along in parallel with the cable axis to leak wave energy propagating in the cable.

2. The leaky coaxial cable of claim 1 wherein said gap means consists of a continuous slot.

3. The leaky coaxial cable of claim 1 wherein said gap means consists of an end-to-end series of spaced slots of the same length which is slightly smaller than the length of the winding reversion of said inner conductor, the space between said slots being positioned at the reversal points of winding of said inner conductor.

4. The leaky coaxial cable of claim 1 wherein said inner conductor is wound on a mandrel of magnetic material or insulating material having magnetic material therein. 

1. A leaky coaxial cable comprising an inner conductor of at least one helically wound conductor with its winding direction reversed alternately at a predetermined interval approximately equal to one-half of the wave length of the operating frequency, and a coaxial outer conductor having gap means extending along in parallel with the cable axis to leak wave energy propagating in the cable.
 2. The leaky coaxial cable of claim 1 wherein said gap means consists of a continuous slot.
 3. The leaky coaxial cable of claim 1 wherein said gap means consists of an end-to-end series of spaced slots of the same length which is slightly smaller than the length of the winding reversion of said inner conductor, the space between said slots being positioned at the reversal points of winding of said inner conductor.
 4. The leaky coaxial cable of claim 1 wherein said inner conductor is wound on a mandrel of magnetic material or insulating material having magnetic material therein. 